Conventionally, MRI apparatuses, etc. are known as an apparatus in which a specimen is placed in a magnetic field (static magnetic field) generated by a magnetic field generator and tomographic images of the specimen is obtained.
FIG. 28 shows a magnetic field generator 1 as an example of the magnetic field generator used in an MRI apparatus. The magnetic field generator 1 includes a pair of plate yokes 3 which are connected with each other by four column yokes 2, to face each other with a space in between. The opposed surfaces of the plate yokes 3 are each provided with a magnetic pole 4. Each magnetic pole 4 includes a permanent magnet group 4a fixed on the opposed surface of the plate yoke 3 and a pole piece 4b fixed on an opposed surface of the permanent magnet group 4a. The permanent magnet group 4a is made of a plurality of unillustrated permanent magnets. Using the permanent magnet group 4a in this way as the source of magnetic field generation enables to reduce running cost as compared to cases in which the magnetic field is generated by supplying electric power to electric magnets. It is also possible to reduce the size of the apparatus since there is no need for an electric power supply apparatus, etc. for driving the electric magnets.
In order to obtain clear tomographic images, the magnetic field generator 1 must be able to generate, within a magnetic field space 5 in its space, a magnetic field which has a uniformity accuracy within 1×10−4 (within 100 PPM) in a range of 0.02 T through 3.0 T. However, the permanent magnet group 4a recently is often made of Nd—Fe—B sintered magnets, which has a residual magnetic flux density temperature coefficient of −0.1%/° C. approx: Magnetic characteristics change with temperature change, so it is difficult to create a uniform magnetic field of a desired intensity. In an attempt to overcome this problem, there is prevailed a technique as shown in FIG. 29, of covering the four column yokes 2 and the pair of plate yokes 3 where the magnetic poles 4 are provided, with a heat insulation member 6 thereby reducing the temperature change caused by changes in ambient temperature in each element (particularly the permanent magnet group 4a) of the magnetic field generator 1.
Also, there is prevailed a technique of employing a heater in addition to the heat insulation member 6 to maintain the permanent magnet group 4a at a constant temperature. As an example, Patent Document 1 for example discloses a technique of providing a surface heater on an inner surface of the heat insulation member 6 and moving the warmed air in the heat insulation member 6 by a fan. Also, Patent Document 2 discloses a technique of providing a surface heater on a surface facing away from the opposed surface in each of the pair of plate yokes 3. Further, Patent Document 3 discloses a technique of providing a surface heater on each side surface of the plate yokes 3. However, the technique according to Patent Document 1 poses a problem of complication in apparatuses related to temperature control since the air must be forced to move by the fan. In addition, use of air as a heat transfer medium poses another problem that the heat generated by the surface heater is not transferred efficiently to the permanent magnet group 4a. The techniques according to Patent Documents 2 and 3 also have the problem of inefficient transfer of heat generated by the surface heater, to the permanent magnet group 4a because the heat diffuses from a surface of the surface heater facing away from the surface that makes contact with the plate yoke 3.
In an attempt to solve these kinds of problems, Patent Document 4 discloses a technique of providing a heater inside the permanent magnet group 4a or the plate yokes 3, etc. The technique according to Patent Document 4 enables to reduce diffusion of heat from the heater to outside.
Patent Document 1: JP-A 63-43649
Patent Document 2: JP-A 63-278310
Patent Document 3: JP-A 8-266506
Patent Document 4: WO 99/65392